The invention relates to a rotary pump having a bearingless motor in accordance with the preamble of independent claim 1.
Magnetically journalled rotary pumps have established themselves in the art for specific applications in which an impeller is floatingly journalled by magnetic forces in the interior of a preferably completely closed pump housing and is driven by a rotating field which is generated by a stator arranged outside the pump housing. Such pumps are in particular advantageous for such applications in which the fluid to be conveyed may not be contaminated, for example for conveying biological liquids such as blood or very pure liquids such as ultrapure water.
In addition, such rotary pumps are suitable for conveying aggressive liquids which would destroy mechanical bearings in a short time. Such rotary pumps are therefore particularly preferably used in the semiconductor industry, for example for conveying mechanically aggressive fluids on the processing of a surface of semiconductor wafers. Chemomechanical polishing processes (CMP, chemomechanical planarization) can be named as an important example here. In such processes, a suspension, usually called a slurry, of typically very fine solid particles and a liquid is applied to a rotating wafer and there serves for the polishing or lapping of the very find semiconductor structures. Another example is the application of photoresist to the wafer or the roughening of surfaces of computer hard disks to prevent an adhesion of the writing/reading heads by adhesion forces, that is, for example, by Vsn der Waals forces.
In this respect, in a bearingless motor, the rotor is often configured in plate shape or ring shape, with in many cases the height of the rotor being less than half the diameter of the rotor.
Such a bearingless motor is disclosed, for example, in WO-A-96/31934 or in another variant also in EP-A-0 900 572. The term bearingless motor within the framework of this application means that the motor is journalled completely magnetically, with no separate magnetic bearings being provided. For this purpose, the stator is configured as a bearing and drive stator; it is therefore both the stator of the electric drive and the stator of the magnetic journalling. For this purpose, the winding of the stator includes a drive winding with a pole pair number p as well as a control winding of the pole pair numbers p±1. A rotating magnetic field can be produced using these two windings which, on the one hand, exerts a torque onto the rotor which effects its rotation and which, on the other hand, exerts a shear force, which can be set as desired, onto the rotor so that the rotor's radial position can be controlled or regulated actively.
Three degrees of freedom of the rotor can thus be actively regulated. The rotor is passively magnetically stabilized, that is not controllably, but rather magnetically stabilized by reluctance forces with respect to three further degrees of freedom, namely its axial deflection in the direction of the axis of rotation and tilts with respect to the plane perpendicular to the axis of rotation (two degrees of freedom.
In particular when the rotor is configured as a plate shaped or ring shaped rotor, the passive magnetic journalling makes high demands on the axial stabilization or on the stabilization with respect to tilts because the rotor only passively magnetically journalled in the axial direction and with respect to tilts only has a small axial stiffness.
So that the axial thrust which occurs and which is mainly caused by the pressure difference present between the inlet and the outlet of the pump does not have to be completely taken up by the axial bearings, very different measures are known in centrifugal pumps to balance the rotor with respect to the axial direction.
The problem of the axial thrust balance is thus particularly serious in pumps with a magnetically journalled rotor, in particular when the axial journalling takes place magnetically via reluctance forces completely without mechanical bearings. To balance the rotor of such a bearingless motor, in addition to the magnetic reluctance force, only construction measures are available which influence the axial position via fluid dynamic compensation forces.
Measures known today for the axial balancing of the rotor for high pump performances or with more highly viscous fluids, such as photoresist or slurry, which can have viscosities of up to more than 100 centipoise, are in particular also often not sufficient with such centrifugal pumps which work in accordance with the principle of the bearingless motor.
There is the problem in the configuration of such rotary pumps as centrifugal pumps that the winding heads of the stator and the outlet or possibly also the inlet of the pump housing impede one another spatially.
To solve this problem, a pump is proposed in WO-A-96/31934 having a so-called temple motor in which the coil cores of the stator each have the shape of an “L”, with the long limb in each case extending parallel to the axis of rotation, whereas the short limb is directed radially inwardly to the axis of rotation. The stator, which is configured as a bearing and drive stator, has two windings, namely the drive winding and the control winding, which are configured as discrete coils and are wound around the long limbs of the L-shaped coil cores. Such a temperature manages without winding heads so that the outlet of the pump housing can be arranged without spatial hindrance at the level of the impeller in the form of a radial outlet passage. This means that it is possible with the temple motor that a radial outlet passage can be arranged at the pump housing so that the drive, more exactly a magnetic stator plane of the stator of the motor, and a center plane of the rotor, coincides with a center axis of the radial outlet passage.
I.e. both the magnetic center plane of the drive and bearing stator and the center plane of the rotor are at the same height as a center axis of the radial outlet passage of the pump housing so that the hydraulic forces which act on the rotor through the drainage of the fluid via the radial outlet passage act symmetrically on the rotor with respect to the axial direction or with respect to the center plane of the rotor. The hydraulic forces acting on the rotor in the axial direction thereby compensate one another so that no additional measures are required for compensating such axial thrust forces. In brief: The temple motor allows an arrangement of the outlet or also of the inlet at the pump housing symmetrical with respect to the center plane of the rotor.
This embodiment as a temple motor, e.g. in accordance with FIG. 12 of WO 96/31934, is, however, subject to the restriction that it has a relatively large space requirement and is complex and/or expensive in design due to the high construction shape. In addition, so-called back-to-back rotor arrangements (see below) are admittedly possible in principle in dependence on the construction shape, but can often only be realized with difficulty, if at all, depending on the specific embodiment of the pump housing, in particular because one of the feeds to the pump housing has to be conducted through the temple of the temple motor.
The mentioned back-to-back arrangements are as mentioned likewise shown in examples in WO 96/31934. The rotor is here equipped, with respect to the axial direction which is equal to the axis of rotation of the rotor, at both sides with conveying means such as vanes so that the rotor is acted on with the pressure of the fluid flowing into the pump housing at both sides and substantially uniformly via two axially mutually oppositely disposed inlet passages so that a resulting axial thrust is thereby substantially prevented. The drive is, however, here only possible by means of a temple motor since otherwise the outlet passages of the pump housing would have to be arranged asymmetrically with respect to the center plane of the rotor disk, which would in turn result in huge hydraulic axial forces onto the rotor via the outlet passages.
Another rotary pump having a bearingless motor is shown in EP 0 900 572 A1 which admittedly dispenses with the temple motor, which is very large and complex from a construction aspect; however, this is obtained at the cost of a very asymmetrical arrangement of the inlet and outlet of the pump housing so that relatively large axial hydraulic forces accordingly arise which can only be compensated at all by specific construction measures up to a specific amount and not in all desired operating states. The areas of use for this kind of pumps are thus correspondingly restricted.
With the exception of the above-described temple motors, a general problem of bearing less motors which are passively magnetically journalled in the axial direction only via reluctance forces, summarized in that the drive, that is the stator of the motor, coincides with a center plane of the rotor. I.e. a center plane of the drive and bearing stator and the center plane of the rotor are at the same height so that at least the outlet passage of the pump housing has to be arranged very asymmetrically with respect to the center plane of the stator, which results in corresponding relatively large axial hydraulic forces which have to be compensated in a complex and/or expensive manner with other means.
In brief: The stator prevents a symmetric arrangement of the outlet or of the inlet with respect to the center plane of the rotor.